Field of Invention
The present invention relates to a liquid crystal alignment agent of a vertical aligned liquid crystal display element. More particularly, the liquid crystal alignment agent includes polyamic acid or polyimide and solvent.
Description of Related Art
There is a requirement of the wide view angle of the liquid crystal display element, thus the requirements of the electrical properties and display properties have become stricter. In wide view angle liquid crystal display element, the vertical alignment liquid crystal display element is widely studied. For meeting better electrical properties and display properties, liquid crystal alignment film becomes one of the important factors.
The liquid crystal alignment film of the vertical alignment liquid crystal display element is used to regularly align the liquid crystal molecules with a larger pretilt angle when the electrical field is not applied. For producing the aforementioned liquid crystal alignment film, a liquid crystal alignment agent having polymers such as polyamic acid or polyimide is firstly coated on a surface of the substrate, being subjected to a thermal treatment and an alignment treatment.
JP Patent publication No. 2009-237545 discloses a polyamic acid polymer applied to form a liquid crystal alignment film in a liquid crystal display element. The polyamic acid polymer is obtained by polymerizing a diamine compound having a structure of formula (VII) and a tetracarboxylic dianhydride compound:

in formula (VII), RI and RIII are respectively an ether group, a thioether group, an ester group or a thioester group, and it does not consider a relative position of the ester group and the thioester group; RII is a methylene group or an alkylene group of 2 to 10 carbons; RIV is a single bond, a methylene group or an ethylene group; X is a monovalent steroid-containing group of 17 to 40 carbons.
The aforementioned liquid crystal alignment film can keep the liquid crystal molecules at 86° of the high pretilt angle to achieve excellent liquid crystal alignment properties. However, when the polyamic acid polymer of the prior art is subjected to the aligning treatment, the liquid crystal alignment agent easily be affected by the conditions of the aligning treatment, so as to lower the aligning ability of the liquid crystal alignment film, further decreasing the pretilt angle uniformity of liquid crystal molecules. Thus, the aforementioned prior art has a defect of process stability. In addition, when the liquid crystal alignment film is applied in the liquid crystal display element, the liquid crystal display element has a defect of reliability after the liquid crystal display element is subjected to high temperature and high humidity testing.
Accordingly, there is a need to improve the aforementioned disadvantages for meeting the requirements of the liquid crystal alignment agent.